A running shoe and a padel shoe are built for opposite loads, which is why one wears out quickly doing the other's job.
The load is sideways
Running is a rolling stride: the foot lands, rolls through, pushes off forwards. Padel is short lateral bursts — a step to the glass, a stop, a push back. The foot pushes against the side of the shoe rather than rolling through it, and it does so hundreds of times in an hour.
Where it fails first
Two places, usually. The outsole goes at the pivot points under the ball of the foot, because that is where the shoe turns against the surface. And the upper gives out at the inside of the forefoot, where the foot pushes outward against material that was not built to be pushed sideways.
Checking those two places tells you more about where a pair is in its life than how the shoe looks from above.
Why the sole pattern matters here
Padel is usually played on sand-dressed artificial grass. A smooth or road-running tread has nothing to clear that sand with, so it rides on top of it. Padel soles are cut in herringbone or omni patterns for that reason, and a sole designed for a clean hard surface is the wrong tool on a sanded court.
Fit and sizing
Lateral load makes fit matter more than it does in a straight line. A shoe that is slightly too long lets the foot slide inside it on every stop, which is felt in the toes and paid for in the upper.
Sizing runs by maker: the same nominal size differs between brands, and the safest comparison is a pair of the same brand already owned. It is also worth fitting late in the day, with the socks actually played in — feet swell over a session, and court socks are thicker than everyday ones by enough to move a borderline fit.
Lacing and heel hold
Lateral load is carried by the whole upper, but it starts at the heel. A heel that lifts on a hard stop lets the foot slide forward inside the shoe, and everything downstream of that — toe pressure, wear on the inside forefoot, a foot that arrives late to the next step — follows from it. Using the top lace eyelet, which many players skip, is what most shoes rely on to hold the heel down.
Two pairs last longer than twice one
Midsole foam recovers between sessions, and it recovers slowly. A pair played on consecutive days is being compressed again before it has come back, and it flattens sooner. Players who train often tend to rotate two pairs for that reason rather than for hygiene.
When to retire a pair
Before the sole goes through, usually. Once the tread at the pivot points is smooth the shoe is no longer gripping the way its pattern intended, even though the upper may look new and the rest of the sole may be untouched. That is the point at which a shoe stops being a padel shoe and becomes a trainer with a good sole pattern printed on it.